1. The Field of the Invention
The present invention concerns the steering of road motor vehicles. More particularly, it relates to an electrical steering system without any mechanical linkage between the steerable wheels and the steering wheel, which will be referred to herein as xe2x80x9celectrical steeringxe2x80x9d for simplicity. This type of steering is very commonly called xe2x80x9csteer by wirexe2x80x9d.
2. The Related Art
In electrical steering, the following means is substituted for the traditional mechanical and/or hydraulic system, which may be assisted or unassisted, existing between the steering wheel and the steerable wheels. At the steerable wheel level, there is an electrical actuator, preferably an individual one for each steerable wheel, the purpose of which is to set the appropriate steering angle for the respective steerable wheel or wheels. The steering operating means available to the driver of the vehicle may be a traditional steering wheel or a lever such as a joystick, or any other suitable device. The commands given by the driver of the vehicle to his operating device are sent to the actuators via an electrical link, with the entire system being placed under the supervision of a computer loaded with programs suitable for driving the actuator or actuators appropriately.
One of the advantages of this technology is that it is ideally compatible with electronics, the advances in which are making increasingly sophisticated feedback control systems possible, so that the steering of the steerable wheels can not only be subject to the manual controls but can also be supervised by a safety system. For instance, the steerable wheels may be set to an angle which not only takes into account the instructions from the driver of the vehicle, but which also takes into account the dynamic parameters observed in the vehicle.
Electronic steering, therefore, has opened up a much wider field for controlling the stability of a vehicle. For example, whereas an automatic system for correcting the direction of the vehicle currently applies corrective yaw movements using the brakes of only one road wheel, changing over to electrical controls for the various functions in a vehicle makes it possible to alter the steering angle of the different steerable wheels of the vehicle in order to correct its trajectory.
However, the steering of a vehicle is a function which is essential and vital to safety, like the brakes. Therefore, in order to be substituted for the assisted or unassisted mechanical steering almost universally employed at present in all road vehicles, it is essential for an electrical steering system to be extremely reliable. This is why electrical systems are generally designed to be redundant, with a view to avoiding critical consequences in the event of a failure. A further requirement is that the greater complexity of the redundant systems must not, in final analysis, lead to such a probability of failure that it is prevented from improving safety.
The present invention provides an electrically controlled steering system which is simple and whose operation is very reliable. Its architecture is redundant for all of the electrical components which are used. In order to control the steering of a steerable wheel of a vehicle, the invention also provides a particular electrical actuator which is well suited to this electrical steering system, in which the actuator comprises a reference end and a control end. The control end is displaced with respect to the reference end by at least three electric motors that act in parallel, each electric motor having its own electrical connection which is independent of that of the other electric motors.
The redundancy principle is based on triplication of certain components, namely the position sensors, the electric motors, and the necessary controllers, as well as the electrical lines connecting the various components in question. This makes it possible to create three control channels which operate in parallel. More precisely, the three control channels operate simultaneously (although not necessarily identically should different softwares be used) and lead to identical actions so long as there is no anomaly. This redundancy, which is referred to as active, makes it possible to ascertain with a high degree of reliability which of the three channels is defective, and makes it possible to continue working with two control channels under conditions which are not significantly downgraded, at least until the vehicle has returned to a region in which the vehicle, and especially its passengers, are in safety.
The steering system for vehicles according to the invention comprises:
at least one steerable wheel capable of being steered;
operating means for effecting a change of direction, which deliver three electrical signals that all carry the same information about the required change of direction;
for each of the steerable wheels, an electrical actuator for altering the steering angle of the steerable wheel, the electrical actuator comprising a reference end and a control end which is displacable with respect to the reference end, the electrical actuator being suitably connected to the steerable wheel and having three electric motors which act in parallel in order to displace the control end with respect to the reference end;
for each electrical actuator, three position sensors for sensing the relative position of the control end with respect to the reference end;
three controllers which operate in parallel, each forming part of an electrical control channel for the steering angle, the controller of each electrical channel receiving one of the three electrical signals and being connected to one of the position sensors and driving one of the electric motors in order to set the steering angle, the torques delivered by the respective motors being added together in normal operation;
at least one interconnection bus for the three electrical control channels; and
means for detecting a discrepancy in the status of one electrical channel with respect to the other two, in order to change to an operation in a downgraded mode in the event of a discrepancy.